Stick for measuring the level of a molten metal bath

ABSTRACT

A measurement rod for measuring the level of a liquid bath is cooled but is provided with a solid end piece so that the bath cannot solidify in front of the orifice of the measurement gas injected in the bath, for which the pressure is being measured.

The invention relates to a rod for measuring the level of a moltenliquid bath.

This type of rod is used to determine the depth of very hot materialbaths that are difficult to access for observation, located incrucibles. The measurement is made by injecting a constant flow gas intothe liquid bath, getting it to pass through a duct inside the rod thatopens up at a lower free end in the liquid bath, and measuring thepressure to be applied to the gas to impose injection at the requiredflow.

A limitation to these measurement rods occurs if the bath temperature ishigh, which is the case in some vitrification applications, since theusual metallic materials used to make the rod lose their strength atthese temperatures. Furthermore, some liquid baths such as baths thatcontain elements such as sulphates, chlorides or molybdenum become verycorrosive. The rods then have to be replaced frequently.

The purpose of this invention is to improve the structure of these levelmeasurement rods to make them capable of resisting liquid baths in whichconditions are too severe for normal measurement rods.

The selected solution consists of cooling the rod by an internal circuitthrough which cold liquid flows. This process is effective in preventingcorrosion and loss of strength, but it is essential to avoid causingsolidification of the bath at the outlet from the gas duct since it willthen be impossible to inject gas from the rod. Heat exchange between themolten bath and a much colder coolant creates intense cooling of themolten bath close to the rod.

The improved structure of the level measurement rod proposed in thispresentation to satisfy these requirements comprises a gas injectionduct into the liquid bath at one free end of the rod, and innovatively,a side casing surrounding a cooling liquid channel, and an end piece atthe free end thicker than the side casing, and solid. The thick andsolid end piece locally reduces heat transfer and helps to keep the bathat a higher temperature in front of it than around the side casing. Thisarrangement keeps the bath in a liquid state, without being too viscous,around the free end of the end piece and therefore maintains gasinjection for the level measurement to be made at the required flow.

Advantageously, the casing is made of metal, contains a sleeveconcentric with it separating a cooling liquid supply channel from acooling liquid evacuation channel, the end piece has an appropriatethickness equal to 35 mm in one particular example, and more generally afew centimetres, and the flow of the cooling liquid is adjusted to coolthe liquid bath to a temperature slightly greater than thesolidification temperature (for example 1000° C.) in front of the endpiece.

We will now describe the invention in more detail with reference to thesingle FIGURE.

The melting bath 1 into which the rod 2 is immersed may be a product tobe vitrified, and may be corrosive at high temperatures. The applicantshave developed a cold crucible technique in which the products to bevitrified are not heated by means of the crucible that contains them,but rather by electromagnetic induction phenomena, while the crucible iscooled and is covered with a solidified layer of the bath material thatis not corrosive. The invention is frequently used in this type of coldcrucibles, but other applications are not excluded.

The rod 2 comprises a cylindrical outer casing 4 finishing on an endpiece 5 at the free end of the rod 2, a gas injection duct 6 extendingalong the centre line of the casing 4, passing through the end piece andfinishing at one orifice 7, and a sleeve 8 between the casing 4 and theduct 6 and parallel to them, that finishes at a short distance from theend piece 5. The end piece 5 is engaged in the end of the casing 4 (asit is shown) or is an extension to it. The cooling liquid supply pipe 9opens up into the vertices of the sleeve 8, and the cooling liquidevacuation pipe 10 opens up into the casing 4. Cooling liquid passesthrough the sleeve 8 towards the end piece 5, in front of which it exitsfrom the sleeve 8 to start counter current circulation between thesleeve and the casing 4 as far as the duct 10.

The casing 4 is thin so that the rod 2 is not too heavy, but the endpiece 5 is solid and is much thicker, its thickness (or height) may forexample be 35 mm, but the value is chosen so as to obtain a temperaturein the molten bath high enough to be able so that the gas flow necessaryfor the measurement can be ejected due to sufficient fluidity of thebath, but also low enough to guarantee good mechanical behaviour andresistance to corrosion at the end of the rod 2.

In the case of a bath of molten material at approximately 1200° C. to1300° C. and a rod made of a metallic material such as Inconel, the rodbeing immersed into the bath by a length of about half a meter, a flowof water at 20° is applied to extract a heat power of 5.3 kW; the bathis then cooled to about 600° C. along casing 4, which solidifies usualbaths. However, although the cooling jet is directed towards the endpiece 5, the end piece is cooled more moderately due to its thickness;the temperature is of the order of 1000° C. on its inside or free face11, such that the bath 1 remains liquid at this point and there is norisk of blocking the orifice 7. Corrosion by the bath 1 at thesetemperatures is moderate. The end piece 5 can be cooled more providedthat the bath 1 remains liquid in front of it.

1. A measurement probe for measuring a level of a molten liquid bath,the probe comprising: an outer casing; an end piece having an orifice,the end piece being disposed at an end portion of the outer casing; anda gas injection tube disposed inside of the outer casing and connectedto the orifice in the end piece, wherein a measurement of the level inthe molten liquid bath is based at least on a pressure measurement of aflow of gas into the liquid bath through the gas injection tube.
 2. Theprobe of claim 1, further comprising: a sleeve disposed inside of theouter casing, said sleeve forming a flow channel for cooling the probe.3. The probe of claim 1, wherein a thickness of the end piece issubstantially larger than a thickness of the outer casing.
 4. The probeof claim 1, wherein the probe is made of metal.
 5. The probe of claim 4,wherein the metal is Inconel.
 6. The probe of claim 2, wherein a flowrate of a cooling liquid flowing through the flow channel is adjusted tocool the liquid bath at a location in front of the end piece to atemperature slightly greater than the solidification temperature of theliquid bath.